Halliday & Matthiessen (1999: 190):
One way of exploring [the primary categories of things] is by noting which participant rôles each category of participant is typically associated with. The critical rôles, in this respect, are those of Senser, Sayer and Actor, operating respectively in figures of sensing, saying and doing-&-happening. When we investigate these, however, we find that the overall categorisation of phenomena that is revealed in this way displays a further dimension of complexity: at the highest level, all phenomena are distributed into two broad experiential realms, the material and the semiotic.